The present invention relates generally to a pulse generator for indicating a change in the magnitude of an input signal, and more particularly to a comparator circuit that generates an output pulse upon either positive or negative transitions of an input signal routed through first and second parallel networks to the positive and negative input terminals of the comparator.
The pulse generator of the present invention may be employed to address changes in the automotive industry relating to cruise control systems. Specifically, most conventional cruise control modules are operated by the driver using at least three switches (resume, set speed, and accelerate). Each switch input is typically provided to the cruise module as an individual battery voltage input signal. In other words, if the cruise module detects a battery voltage on an input line corresponding to the resume switch, then the module carries out commands associated with the driver""s intent to resume cruise.
There is, however, an increasing demand for analog switch inputs to cruise modules wherein all of the switch signal inputs are provided over a single input line. Each switch, when activated by the driver, results in a different voltage level on the input line to the cruise module. Accordingly, in order to carry out the driver""s intent, the cruise module must interpret this voltage level as corresponding to a particular switch.
An A/D converter could be employed to decode the input switch voltage level, but the cost of such converters and associated electronics, as well as the relatively limited opportunities for expanded functionality, make such an approach undesirable. Alternatively, a plurality of staged comparator circuits could be used to decode the input voltage level. Comparator thresholds could be set such that only one comparator will produce an output signal for each of the possible switch input voltage levels. This approach is undesirable because of the relatively large number of components required for its implementation.
A more desirable modification to the conventional cruise module is to employ a switch processing microcontroller (with an on-board A/D converter) to decode the analog switch inputs. While switch processing microcontrollers are relatively expensive, the increased functionality of a microcontroller in addition to the primary microcontroller of the cruise module, and expanded flexibility for future designs outweigh the cost disadvantages.
In addition to increased cost, however, a second microcontroller results in increased radiated emissions due to clocking pulses associated with the microcontroller. Thus, the second microcontroller should include a xe2x80x9csleepxe2x80x9d mode function, wherein the microcontroller essentially shuts down until it is needed to decode a switch input, thereby reducing radiated emissions. Accordingly, a circuit is needed for generating a xe2x80x9cwakexe2x80x9d signal (to activate the second microcontroller) each time the switch input voltage to the cruise module changes.
The present invention provides a pulse generator for indicating a change in the magnitude of an input signal, such as the analog switch input to a cruise control module, by outputting a pulse which may be used, for example, as a wake signal to activate a switch processing microcontroller as described above. The pulse generator includes a comparator, a first network connected to one input to the comparator, and a second network connected to the other input to the comparator. Each of the first and second networks are capacitively coupled to the analog switch input signal, and include a voltage divider network to scale the input signal. The second network further includes a low pass filter to slow the response of the second network to changes in magnitude of the input signal. Accordingly, the signal outputted by the first network to the first input of the comparator deviates from a baseline magnitude upon the occurrence of an input signal magnitude change more quickly and to a greater extent than does the signal outputted by the second network to the other comparator input.
Thus, when one of the cruise control switches is actuated, the analog switch input signal provided to the first and second networks drops, for example, from a first voltage to a second voltage. The response characteristics of the first and second networks are designated such that the first input to the comparator drops more quickly, and below the signal present at the second input to the comparator. If the first input is the positive input to the comparator, the pulse generator will output a pulse (the wake pulse) with a duration equal to the time during which the first input is less than the second input. Since the analog switch input signal is present at the cruise module input for a period of time which is greater than the duration of the wake pulse, the second microcontroller, once activated, can decode the analog switch input signal. After the input signal is decoded, the second microcontroller re-enters the sleep mode until another switch transition occurs. As such, the second microcontroller remains inactive except when it is needed to decode the analog switch input signal, thereby reducing the radiated emissions of the second microcontroller.